There is a plethora of information on the Internet. Just because it's on the Internet though, doesn't mean that it's yours for the taking. When you use Internet sources in your formal writing, give credit where credit is due.
There are quite a few styles to use for citing sources. The most commonly used are APA, MLA, or Chicago style. Below are examples of citing Internet sources in these three styles.
APA
Kehoe, Brendan P. (January 1992). Zen and the Art of the Internet:
A Beginner's Guide to the Internet, First Edition [Electronic
version]. Retrieved September 5, 2007, from
http://www.cs.indiana.edu/docproject/zen/zen-1.0_toc.html
MLA
Kehoe, Brendan P. Zen and the Art of the Internet: A Beginner's
Guide to the Internet, First Edition [Electronic version].
January 1992. 5 September 2007
http://www.cs.indiana.edu/docproject/zen/zen-1.0_toc.html.
Chicago
Kehoe, Brendan P. Zen and the Art of the Internet: A Beginner's
Guide to the Internet, First Edition, January 1992.
<http://www.cs.indiana.edu/docproject/zen/zen-1.0_toc.html>
accessed September 5, 2007).
Even your informal writing should have a citation for your Internet sources. If you use information from a website, be sure to note the website and its url within what you write. For instance, the glossary for this tutorial was modified from terms listed at Whatis.com. At the bottom of the glossary, Whatis.com is given credit for these entries.